1. Field of the Invention
The invention is related to the field of nuclear magnetic resonance ("NMR") apparatus and methods. More specifically, the invention is related to methods for conducting NMR measurements in a manner which optimizes the use of electrical power by the NMR instrument.
2. Description of the Related Art
NMR instruments adapted for well logging can be used for determining, among other things, the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space of earth formations. Methods for using NMR well logging measurements for determining the fractional volume of pore space and the fractional volume of mobile fluids are described, for example, in, Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex. (1990).
NMR well logging instruments known in the art are typically designed to make measurements corresponding to an amount of time for hydrogen nuclei present in the earth formation to realign their spin axes, and consequently their bulk magnetization, either with an externally applied static magnetic field, or perpendicularly to the magnetic field, after momentary reorientation of the nuclear spin axes. The externally applied magnetic field is typically provided by a permanent magnet disposed in the NMR instrument. The spin axes of the hydrogen nuclei in the earth formation, in the aggregate, become aligned with the static magnetic field induced in the earth formation by the permanent magnet. The NMR instrument also includes an antenna positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces a corresponding RF magnetic field in the earth formation in a direction orthogonal to the static field induced by the permanent magnet. This RF pulse (called an "A-pulse" hereafter) has a duration and amplitude selected so that the spin axes of the hydrogen nuclei generally align themselves perpendicular both to the RF magnetic field and to the static magnetic field. After the A-pulse ends, the nuclear magnetic moment of the hydrogen nuclei gradually "relax" or return to their alignment with the static magnetic field. The amount of time taken for this relaxation is related to the properties of interest of the earth formation.
Also after the A-pulse ends, the antenna is typically electrically connected to a receiver, which detects and measures voltages induced in the antenna by precessional rotation of the spin axes of the hydrogen nuclei. While the hydrogen nuclei gradually realign their spin axes with the static magnetic field, they do so at different rates because of inhomogeneities in the magnet's field and because of differences in the chemical and magnetic environment within the earth formation. Different rates of realignment of the spin axes of the hydrogen nuclei result in a rapid decrease in the voltage induced in the antenna. The rapid decrease in the induced voltage is referred to as the free induction decay (FID).
After a predetermined time period following the FID, another, longer RF pulse (called a "B-pulse" hereafter) is applied to the antenna. The B-pulse has a duration and amplitude selected to reorient the spin axes of the hydrogen nuclei in the earth formation by an axial rotation of 180 degrees from their immediately previous orientations. After the B-pulse, hydrogen nuclear spin axes that were realigning with the externally applied field at a slower rate then are positioned so that they are "ahead" of the faster realigning nuclear spin axes. This causes the faster realigning axes to be positioned "behind" the slower realigning spin axes. The faster realigning spin axes then eventually "catch up" to, and come into approximate alignment with, the slower aligning spin axes at some time after the B-pulse. As a large number of the spin axes become aligned with each other, the hydrogen nuclei again are able to induce measurable voltages in the antenna. The voltages induced as a result of realignment of the hydrogen nuclear spin axes with each other after a B-pulse is referred to as a "spin echo". The voltage induced by the spin echo is typically smaller than the original FID voltage induced after cessation of the A-pulse, because the aggregate nuclear axial alignment, and consequently the bulk magnetization, of the hydrogen nuclei at the time of the spin echo is at least partially realigned with the static magnetic field and away from the sensitive axis of the antenna. The spin echo voltage itself rapidly decays by FID as the faster aligning nuclear axes again "dephase" from the slower aligning nuclear axes.
After another period of time equal to two of the predetermined time periods between the A-pulse and the first B-pulse, another B-pulse of the same amplitude and duration as the first B-pulse can be applied to the antenna. This next B-pulse again causes the slower realigning spin axes to be positioned ahead of the faster realigning axes, and eventually another spin echo will induce voltages in the antenna. The voltages induced by this next spin echo will typically be smaller those induced by the previous spin echo.
Successive B-pulses are applied at regular time intervals to the antenna to generate successive spin echoes, each one typically having a smaller amplitude than the previous spin echo. The rate at which the peak amplitude of the spin echoes decreases is related to the properties of interest of the earth formation, such as the fractional volume of pore space or the fractional volume of mobile fluid filling the pore space. The number of spin echoes needed to determine the rate of spin echo amplitude decay is related to the properties of the earth formation. In some cases as many as 1,000 spin echoes may be needed to determine the amplitude decay corresponding to the particular formation properties of interest.
A limitation of NMR well logging instruments using the just-described RF pulse sequence is that this pulse sequence uses a very large amount of electrical power. Typically the DC power requirement for the NMR logging instruments known in the art is about 1 KW; the peak power required for effective nuclear excitation can be as high as 30 KW in each pulse. As is known in the art, a typical well logging cable has a power transmission capacity of about 1.5 KW. Using NMR pulse sequences known in the art it is impractical to increase the RF power in order to improve signal to noise or to increase the axial speed ("logging speed") at which the instrument is moved through the wellbore (the increased speed being desired by the wellbore operator to save operating time and associated costs). It is also impractical to combine NMR well logging instruments using pulse sequences known in the art with other well logging instruments because the NMR logging instrument uses nearly the entire power transmission capacity of the typical well logging cable.